1. Field of the Invention
The invention relates to a high temperature protective coating made from an alloy containing nickel, cobalt, chromium, aluminum and also yttrium, especially for structural elements of an austenitic material.
2. Description of the Prior Art
Such high temperature protective coatings are used especially when the basic material of structural parts made of heat resistant steel and/or alloys used at temperatures above 600.degree. C must be protected.
By use of these high temperature protective coatings it is intended to slow or completely stop the effects of high temperature corrosion, caused especially by sulfur, oil residue, oxygen, alkaline earths and vanadium. Such high temperature protective coatings are prepared such that they can be applied directly to the basic material of the structural element to be protected.
High temperature protective coatings are of special importance with structural elements of gas turbines. They are mainly applied to rotors and guide vanes as well as heat accumulation segments of gas turbines.
To manufacture these structural elements, an austenitic material on the base of nickel, cobalt or iron is preferably used. Nickel super alloys are mainly used as basic material in the manufacture of structural parts of gas turbines.
Structural parts intended for gas turbines are, for example, provided with protective coatings made by an alloy containing nickel, cobalt, chromium, aluminum and yttrium.
The aluminum content of these alloys is relative high, while the chromium content is very low, which leads to a low corrosion resistance. This is due to the low chromium content.
Protective coatings made of the alloys mentioned above have a tendency to form a covering layer containing aluminum oxide on their surfaces under operational condition, especially if exposed to temperatures of more than 900.degree. C. Because of the yttrium contained in the alloy, a certain adherence of the aluminum oxide covering layer to the protective coating occurs.
The structure of these protective coatings consists of a matrix into which an aluminum-containing phase has been inserted. Quick aluminum depletion of the areas near the surface results because of continuous oxidation. This leads to an increased susceptibility to corrosion of the protective coatings.
As another disadvantage it must be emphasized that these protective coatings are not sufficiently adapted to the basic material of the structural elements to be protected. Especially at high temperatures adherence is not sufficient.